In cellular wireless communicational networks, or "cellular networks," a served area is divided into cells. Each cell is further divided into sectors, except in the case of omni-directional cells, in which the entire cell comprises a single sector. Each cell is served by at least one base station located at a cell site typically at the center of the cell. All of the base stations are connected to a message switching center ("MSC") via a base station controller ("BSC") and hardware links. A plurality of mobile units are connected to the MSC by establishing radio links with one or more nearby base stations.
In other cellular telephone technologies, such as time division multiple access ("TDMA"), as a mobile unit travels from one cell to another, the radio link between the mobile unit and the base station serving the first cell has to be broken and then replaced by a radio link between the mobile unit and the base station serving the second cell. In contrast, in a code division multiple access ("CDMA") cellular telephone system, because the same frequency band is used for all cells and sectors, the first link need not be broken before connecting with the second link. As described in greater detail below, this process is referred to as a "soft handoff" or "softer handoff." The CDMA waveform properties that provide processing gain are also used to discriminate between signals that occupy the same frequency band. A mobile unit thus need not switch frequencies when a call is transferred from one cell or sector to another. Additional details regarding the specifics of the CDMA cellular telephone environment are described in TIA/EIA/IS-95-A, Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System (hereinafter "CDMA Standard"), which is hereby incorporated by reference in its entirety.
In the context of a cellular telephone system, "handoff" is the process of handing over a call from one sector to another when a mobile unit detects that acceptable communication with the other sector is possible. This occurs mainly when the mobile unit nears a sector boundary or the current communication link is weakened by radio frequency ("RIF") shadowing and another potential communication path from another sector is enhanced. In general, handoff consists of three phases. During the first phase, referred to as "handoff initiation," the handoff process is triggered. During the second phase, referred to as "target selection," a determination is made which sectors are candidates for receiving the handoff. During the third and final phase, referred to as "handoff completion," the mobile unit is transferred from the old sector to the new sector.
The term "soft handoff" is commonly used to refer to a handoff in which the mobile unit commences communication with a new base station without interrupting communications with the old base station, i.e., the call is maintained on both base stations. If there are three cells involved in the handoff, the call will be maintained by all three base stations. A "softer handoff" refers to a handoff in which the call is maintained on one base station for different sectors of the same cell. It will be recognized by those skilled in the art that, for various reasons, softer handoff typically requires less transmit power than soft handoff on both the forward (i.e., base station-to-mobile unit) and reverse (i.e., mobile unit-to-base station) links.
In order to optimize the performance of a CDMA network, a number of factors must be considered. Arguably, the most important of these are network capacity, that is, the number of calls that can be handled by the network at a given time, and drop call probability, that is, the probability that a call will be dropped during active communication, as well as the relevant interaction of the foregoing factors with network resources, such as transmit power, handoff, and others. Clearly, it is desirable to maximize capacity while minimizing drop call probability.
In CDMA, capacity is soft, i.e., the number of users can be increased; however, as the number of users increases, service is degraded. Capacity of a CDMA system can be increased by minimizing handoff and minimizing the average forward link power required to sustain adequate communication between the mobile unit and the base station. Clearly, however, minimization of these parameters typically increases frame error rates ("FERs") and increases the probability of a call being dropped.
There are a number of methods for increasing the capacity of a CDMA network in areas in which there is a high demand for service. For example, an additional channel could be added; however, even assuming that additional frequency is available, which may not always be the case, this solution is an expensive one for the service provider. Alternatively, a large number of small cells could be deployed. This, too, is an expensive solution, as the service provider must purchase the equipment necessary for implementing base stations at each cell site in addition to the real estate on which the base stations are located.
Yet another method of maximizing the capacity of a CDMA network is to maximize the capacity of each cell thereof by increasing the number of sectors comprising that cell. For example, the capacity of an omni-directional (i.e., single-sectored) cell is X calls. In contrast, the capacity of a three-sectored cell is estimated to be approximately 2.6X, while the capacity of a six-sectored cell is estimated to be approximately 4.8X. In heavy traffic areas of a CDMA network, it is often more cost effective to use one or two sectorized cells instead of a number of smaller, omni-directional cells to provide the necessary amount of coverage.
The reverse link capacity of a CDMA cell/sector may be estimated using the following equation: EQU N=(W/R)*(1/(E.sub.b /N.sub.o))*(1/v)*F*G
where: N=the number of users per sector;
W=spread spectrum bandwidth; PA1 R=data rate; PA1 E.sub.b /N.sub.o =bit energy/noise spectral density; PA1 v=voice duty cycle; PA1 F=frequency reuse factor; and PA1 G=antenna sectorization gain.
As illustrated by the foregoing equation, an important factor to be considered is referred to as the frequency reuse factor. Frequency reuse factor is the ratio of interference from mobile units within a sector to the total interference from all sectors and is calculated using the following equation: EQU F=N.sub.ic /(N.sub.ic +N.sub.oc)
where F is the reverse link frequency reuse factor, N.sub.ic is the in-cell/sector interference, and N.sub.oc is out-of-cell/sector interference. Clearly, as N.sub.oc approaches zero, F approaches one; therefore, the goal is to minimize N.sub.oc, thereby to maximize F.
The capacity of a network can also be increased through the use of directional antennas at the cell sites. A directional antenna reduces the interference seen at the base station because it only receives in the direction of the antenna. In fact, if there were no side- or back-lobes in the directional antenna, the total interference observed by a sector from other sectors would be reduced by a third. Clearly, sectorization through use of directional antennas increases the number of users, it reduces the frequency reuse factor because of the side- and back-lobes of a directional antenna. Hence, as F is marginally reduced during sectorization, G is increased, translating into an overall increase in capacity. Other factors that play an important role in reducing the frequency reuse factor of a network include a uniform, as opposed to haphazard, cellular layout, antenna beamwidth, side- and back-lobe leakage, and whether the antennas are uniform (i.e., all 60.degree. as opposed to some 60.degree. and some 90.degree.).
FIG. 1 illustrates a three-sectored cell embodiment of a CDMA network. In FIG. 1, inter-cell boundaries are represented by solid lines, while inter-sector boundaries are represented by dashed lines. Sectors are represented in FIG. 1 by unit hexagons. Table I below sets forth estimates of the frequency reuse factor and frequency reuse factor ratio for a three-sectored cell embodiment of a CDMA network as shown in FIG. 1 for various antenna beamwidths, where "frequency reuse factor ratio" is calculated by dividing the frequency reuse factor of the illustrated embodiment by that of a CDMA network comprising omni-directional cells (typically 0.62):
TABLE I ______________________________________ Antenna 3 dB Frequency Reuse Frequency Reuse Beamwidth Factor Factor Ratio ______________________________________ 60 0.604 0.974 70 0.591 0.953 80 0.573 0.924 90 0.556 0.897 100 0.536 0.865 110 0.515 0.831 120 0.492 0.794 ______________________________________
The sectorization of cells into six sectors is well known and at present, there are at least two known cellular layouts for six-sectored cells, including a parallelogram cellular layout, as shown in FIG. 2, and a much less common triangular cellular layout, as shown in FIG. 3. As in FIG. 1, in FIGS. 2 and 3, inter-cell boundaries are represented by solid lines, while inter-sector boundaries are represented by dashed lines.
Table II below sets forth estimates of the frequency reuse factor and frequency reuse factor ratio for a parallelogram cellular layout of a six-sectored cell embodiment of a CDMA network as shown in FIG. 2 for various antenna beamwidths, where "frequency reuse factor ratio" is calculated by dividing the frequency reuse factor of the illustrated embodiment by that of a CDMA network comprising omni-directional cells (typically 0.62):
TABLE II ______________________________________ Antenna 3 dB Frequency Reuse Frequency Reuse Beamwidth Factor Factor Ratio ______________________________________ 30 0.550 0.887 40 0.523 0.844 50 0.481 0.776 60 0.442 0.713 ______________________________________
Finally, Table III below sets forth estimates of the frequency reuse factor and frequency reuse factor ratio for a triangular cellular layout of a six-sectored cell embodiment of a CDMA network as shown in FIG. 3 for various antenna beamwidths, where "frequency reuse factor ratio" is calculated by dividing the frequency reuse factor of the illustrated embodiment by that of a CDMA network comprising omni-directional cells (typically 0.62):
TABLE III ______________________________________ Antenna 3 dB Frequency Reuse Frequency Reuse Beamwidth Factor Factor Ratio ______________________________________ 30 0.565 0.911 40 0.539 0.869 50 0.499 0.805 60 0.456 0.735 ______________________________________
As can be seen with reference to Tables II and III above, the manner in which sectors are laid out affects the frequency reuse factor of a CDMA network, thereby ultimately affecting its capacity. For example, for an antenna beamwidth of 30.degree., the frequency reuse factor and frequency reuse factor ration for a sector in the CDMA network shown in FIG. 2 are, respectively, 0.550 and 0.887, while the same parameters for a sector in the CDMA network shown in FIG. 3 are, respectively, 0.565 and 0.911. In terms of frequency reuse factor and capacity, the triangular cellular layout is more optimal than the parallelogram cellular layout.
Referring to FIG. 2, in the parallelogram cellular layout, it will be recognized that each point X is equidistant from three base stations; therefore, it is highly likely that at each point X, there will be no single dominant sector. Instead, there will be six sectors of approximately equal strength (or weakness) and a mobile unit located at a point X will necessarily be in a higher (e.g., four-, five-, or six-way) handoff state. Due to the lack of a single dominant sector, the strengths of each sector are lower; consequentially, performance is degraded. In FIG. 3, in the triangular cellular layout, at each point Y there are four sectors of approximately equal strength. This is an improvement over the parallelogram cellular layout, but still not optimal.
It will be recognized by those skilled in the art that forward link capacity of a CDMA network is directly affected by handoff state, in that a higher handoff state consumes greater resources. In addition, higher handoff states are due to higher out-of-cell signal power, which means that if there is excessive signal interference on the forward link, then capacity is sacrificed and greater forward link transmit power is required to sustain communications between the mobile unit and the base station.
As used herein, unless otherwise specifically designated as "forward link" or "reverse link," "capacity" refers to the overall capacity (i.e., number of calls that can be serviced at one time) of a network.
Other parameters that are desirable to optimize are the average forward link and reverse link transmit powers required per user, as well as the forward link and reverse link frame error rates ("FERs"), all of which are affected by interference and can be improved by reducing the interference seen by a call.
As evidenced by the above, the particular arrangement of the sectors within a six-sectored cell CDMA network will affect the performance of the network.
Accordingly, what is needed is an improved cellular layout for CDMA networks having six-sectored cells that optimizes the capacity of the CDMA network.